Whatever the eventual conclusion to the hunt for the Boston marathon bombers, law enforcement experts as much as social media gurus will be analysing the role played by community websites in assisting – or perhaps hindering – efforts to identify and track down those responsible.

Debate on the Boston blasts has centred on a pair of hugely popular but quite technologically basic bulletin board-type sites, Reddit and 4chan. Before Tamerlan and Dzhokhar Tsarnaev were identified, users of both pored endlessly and publicly over photographs and videos of the packed crowd at the marathon finish, hunting for clues.

There was a particular mass of this on Reddit, which operates a system in which readers' votes push items up a ranking list. Much of this involved crudely computer-drawn circles highlighting people in the marathon crowd who happened to be carrying a rucksack or, to a Reddit user's eye, looked suspicious in some other way. It seemed that being non-white was often a cause for suspicion.

The efforts of these "self-deputised internet detectives", as one journalist called them, saw unwelcome attention focus on several people who, it soon emerged, had nothing to do with the bombings. Some of the speculation bordered on the farcical. One bag-toting presumed suspect, named "Blue Robe Guy" by users, was accused of such actions as "trying to look nonchalant".

Even when authorities issued pictures of the Tsarnaevs, without names, the potentially harmful tide of misinformation did not stop. Reddit users quickly, and wrongly, identified the younger of the pair as Sunil Tripathi, a reportedly depressed Brown University student who disappeared in March.

There was a similar mass of fevered speculation at 4chan, a pointedly anarchic site which began as a discussion point for Japanese animation. The grandly-styled "4chan think tank", a mass of photographs and videos of the bomb scene with people highlighted for reasons such as looking in a different direction to everyone else, was so clearly ludicrous it soon spawned a parody on the very same website.

Amid the chaos, however, there were signs of both self-regulation and also the sheer power of information on such an industrial scale. Reddit users reminded others about Richard Jewell, the security guard falsely accused of involvement in the 1996 Atlanta Olympics bombing. When the FBI circulated photos of the Tsarnaevs, moderators began deleting images of anyone else. Posters then began tracking the brothers' progress using their own photos, including a chilling image of the white-hatted Dzhokhar Tsarnaev standing just yards from the youngest victim of the blasts, eight-year-old Martin Richard.

It is undeniable that, in some instances, crowdsourcing can be hugely powerful. Earlier this year a Reddit user posted a photo of a car's front headlight, the only clue left at the scene of a hit-and-run. A knowledgeable reader was able to pin down not just the model but the precise year.

The greatest paradox is that the most significant single clue for police seems to have come from a very old-fashioned piece of detective work: first-hand testimony from a brave eyewitness. One of the iconic images of the bombs' aftermath was Jeff Bauman, 27, being taken away in a wheelchair, the bottom half of his legs shattered. As soon as Bauman awoke from sedation, his brother has since recounted, he was able to give FBI investigators a vital description of a man who dropped a bag at his feet shortly before the explosion.

Police face "a double edged sword" in dealing with the mass of information after such a public crime, said Peter Kirkham, former detective chief inspector with the Metropolitan police.

The benefits come when police can release images of a particular suspect and use the internet and social media as a gigantic wanted poster. The difficulties arise beforehand, he said: "Unless you see someone doing something you know is part of the act of the crime, you cannot say, 'That's my suspect.' You will not see suspicious behaviour from video footage, let alone still pictures. People have their own pet theories, based on racism or another prejudice, as to why something is suspicious. That's how names get out, which devastates people's lives and distracts law enforcement."